1. Field of Invention
This invention is concerned with an identification system involving an interrogator and coded tags, and more particularly with such a system involving dual frequency tags.
2. Description of the Prior Art
Many identification systems are known which involve a transmitting and receiving interrogator transmitting a microwave or other radio frequency signal and one or more tags, attached to an object to be identified, which re-radiate the transmitted frequency either unmodified or modified to some degree. One of the more common type identification systems is that found in retail establishments where clothing offered for sale by the store is tagged to prevent theft. The tag consists of a frequency doubler, which is usually nothing more than a diode operating non-linearly. The interrogator continuously transmits frequency A and when a tag is in the vicinity of the interrogator, the interrogator receives, re-radiated from the tag, frequency 2A.
In more sophisticated systems, the tag is coded in some way to uniquely identify the tagged object, a vehicle, for example. Such tags alternately modulate and not modulate the interrogating signal to thereby return a unique code to the interrogator. The tendency of such systems to fluctuate widely in amplitude makes the likelihood of receiving incorrect codes quite high.